In an aircraft, and especially a military aircraft, it is often beneficial to determine the location of radio frequency (RF) signal sources associated with active radar weapons systems because these signal sources are a direct threat to accomplishing a mission and aircraft survival. The most common example of such an RF signal source is a radar installation; however, other RF signal sources may be applicable, including targeting systems.
A device that is beneficial in the targeting and identification of threats or objects in the vicinity of the aircraft is the laser range finder (LRF). This device can determine the range to a target by lazing a laser signal at the target, and deciphering a return signal. Based on the characteristics of the return signal, the exact distance to the target can be determined. The disadvantage of the LRF is its necessity to transmit a signal in order to determine useful information such as line-of-sight range to the target. Again, because a signal is transmitted, other devices or aircraft in the area can detect the presence of the LRF by picking up its transmitted signal. In many applications this is an undesired occurrence.
In order to passively detect a radio frequency signal source, a single axis radio frequency interferometer has been used. This interferometer receives RF signals on a linear array of sensors, and subsequently, calculates an angular relationship between the array sensors and the source of RF energy. More specifically, the array of sensors are arranged in a linear fashion and are sensitive to radio frequency signals. The detected signals are then compared to one another, and phase differences in the detected signals can be suggestive of the angle of arrival of the detected RF signal. The single axis RFI is generally attached to an airframe in a known manner, thus allowing angle of arrival determination for the RF signal as it relates to the airframe coordinates.
The single axis RFI has a number of undesirable characteristics inherent in its design. Most importantly, the single axis RFI is limited to a single axis of operation; therefore, the angle of arrival merely defines a plane in which the RF signal source could exist. While this information is of some benefit to a pilot, it still requires further normal processing and further searching for such RF signal sources. As the operation of an aircraft keeps the pilot very busy, especially in a hostile environment, it is undesirable to require the pilot to do more work than is necessary. Consequently, this extensive searching and interaction by the pilot is undesirable.
Another problem inherent in the single axis system is the occurrence of a phenomenon known as the "conic effect." The conic effect detrimentally effects the accuracy of a single axis RFI when the RF signal source is above or below the plane normal to the axis of the array of sensors. When the RF signal source is above or below the plane normal to the array, the RFI sensors detect the RF source as being at an angle closer to perpendicular than is actually the case. This conic effect causes distortions and inaccuracies in the detected angle of arrival of the RF signal. Consequently, the use of the single axis RFI to determine the position of an RF signal source is not necessarily accurate, nor reliable, and many times requires further interaction by the pilot to estimate for correction factor.
Another method by which the single axis RFI system can be used to generate course position and range of an RF signal source is through the use of triangulation. By accumulating two or more "angles" associated with known headings and positions of an aircraft, triangulation can be used to calculate the position of the RF signal source. This method of RF signal source calculation is a very slow and involved process. It is necessary to accumulate two or more data points, which requires a fair amount of time. Furthermore, very involved and time consuming numerical calculations are required to determine the RF signal source position. Lastly, since the single axis RFI suffers from the conic effect, this will detrimentally effect the accuracy of the triangulation process. Consequently, the use of this triangulation process to determine the actual position of a source of RF energy is slow, inaccurate and non-reliable.
The use of single axis RFI, or other locating systems, provides limited amounts of information. In many situations it would be more helpful if the pilot were to know the actual location and geographic characteristics of the RF signal source. While the pilot may be able to visually locate and estimate position of the RF source, such RF signal sources may be located behind a geographic obstacle, such as a mountain. The pilot, not knowing the existence of this mountain or geographic obstacle, will have inaccurate information regarding the actual position of the RF signal source.
Lastly, since the information received by the single axis RFI system is somewhat limited, this information is of little use for other platforms operating in conjunction with the platform utilizing the single axis RFI.